In modern cellular radio systems, a radio network has a strict control on the behavior of a user equipment, UE. Uplink transmission parameters like frequency, timing, and power are regulated via downlink DL control signaling from the base station to the UE.
At power-on or after a long standby time, the UE is not synchronized in the uplink. A first step in accessing the network is therefore to obtain synchronization to the network. This is usually done by the UE by listing to downlink signals and obtain from this signals downlink timing synchronization, an estimate of the frequency error, and also an estimate of the DL path loss. Even though the UE is now time-synchronized to the DL, signals transmitted by the UE are still not aligned with the desired reception timing at the base station due to an unknown round trip timing. Therefore, before commencing traffic, the UE has to carry out a Random Access (RA) procedure to the network. After the RA, eNodeB can estimate the timing misalignment of the UE uplink and send a correction message. The random access procedure may also be used by synchronized UEs without valid uplink allocations for data transmission, in order to request such allocations.
Usually, a Physical Random Access Channel (PRACH) is provided for the UE to request access to the network. An access burst is used which contains a preamble with a specific sequence with good autocorrelation properties. The PRACH can be orthogonal to the traffic channels. For example, in GSM a special PRACH slot is defined.
Because multiple UEs can request access at the same time, collisions may occur between requesting UEs. Therefore LTE defines multiple RA preambles. A UE performing RA randomly picks a preamble out of a pool and transmits it. The preamble represents a random UE ID which is used by the eNodeB when granting the UE access to the network.
The eNodeB receiver can resolve RA attempts performed with different preambles and send a response message to each UE using the corresponding random UE IDs. In case that multiple UEs simultaneously use the same preamble a collision occurs and most likely the RA attempts are not successful since the eNodeB cannot distinguish between the two users with the same random UE ID. In LTE 64 preambles are provided in each cell. Preambles assigned to adjacent cells are typically different to insure that a RA in one cell does not trigger any RA events in a neighboring cell. Information that must be broadcasted is therefore the set of preambles that can be used for RA in the current cell.
The power used by the UE to transmit a RA preamble is typically calculated via open-loop power control. The UE measures the power on some downlink signals with known transmit power—e.g. reference signals or synchronization signals—and calculates the DL path loss. The power of the signals used to estimate the path must be known, therefore this information must be signaled to the UE, either via broadcasted in initial access or possibly via dedicated signaling in handover.
The path-loss is calculated asPL=PRS,RX−PRS,TX where PRS,RX and PRS,TX are the received and transmitted power in dBm of the signal used for path-loss estimation, respectively.
In order to maintain a certain quality criteria for RA reception a minimum signal to noise (interference) ratio at the base station is required. The base station is aware of the present interference situation and can thus calculate the minimum required signal power P0,RACH the RA signal must have at the base station to fulfill the required quality criteria. This power level is also signaled to the UE. Using this power level together with the previous calculated path loss the UE now calculatesPRACH=min{P0,RACH−PL+(N−1)ΔRACH,Pmax},which is the transmit power needed to achieve the power level P0,RACH at the base station. This implies that the path loss—which has been calculated in the DL—is the same for the UL, which typically is not the case for FDD systems. Therefore open loop power control is a rather coarse mechanism. To overcome this limitation very often power ramping is applied. Here each subsequent attempt is performed with a by ΔRACH increased transmission power. In above formula this is reflected by the term
(N−1)·ΔRACH, where N is the transmission attempt number.
The interference level at eNodeB and thus also the required target receive power P0,RACH depends on many factors and can vary over a wide range. Typically, P0,RACH is encoded and transmitted with a rather low number of bits—e.g. 4 bit—and spans around a range of 30 dB.
It is expected that LTE systems will be deployed in a wide range of scenarios, from pico cells to very large cells of up to 100 km and beyond. Since the RA is the first procedure performed by the UE to access the network it is of vital importance that random access works in all anticipated scenarios. If RA fails the UE cannot access the network.
In order to insure satisfactory RA performance the LTE standard defines multiple preamble formats. For the FDD mode four preambles axe defined, the TDD mode even specifies an additional fifth preamble. Some of these preambles are designed for larger cells and are thus longer than other preambles. The reception power and consequently the performance of the random access procedure are affected by RA configuration.